The subject matter of the invention is a method for the preparation of mercaptoalkylsilanes, in which halogen alkylsilanes are reacted with thiourea and ammonia and then the guanidine hydrochloride that forms is separated.
Of the known methods for the preparation of mercaptoalkylsilanes, one procedure has proven technically feasible, which consists in reacting halogen alkyl silanes with thiourea and ammonia (cf. German Fed. Pat. No. 20 35 619). At the end of the reaction, depending on the halogen alkyl silane used, a two-phase system forms: a lower, heavier phase forms containing mostly the guanidine hydrohalide along with varying amounts of the desired reaction product, and an upper phase containing mostly the mercaptoalkylsilane plus varying amounts of guanidine hydrohalide. The amounts of mercaptoalkylsilane in the lower phase constitute a considerable source of loss; on the other hand, considerable problems arise out of the guanidine hydrohalide content in the mercaptoalkylsilane raw product in the upper phase when it comes to working up the product by distillation. These problems consist, on the one hand, in the fact that the guanidine hydrohalides which distill in the mercaptoalkylsilanes and crystallize in part in the tubes of the columns result in severe impairments of the distillation process. Another disadvantage consists in the fact that turbidity occurs in the mercaptoalkylsilanes produced by the distillation due to the fine precipitation of guanidine hydrohalides, and refuses to go away.
The problem therefore existed of conducting the further processing of the raw product produced in the reaction of halogen alkyl silanes with thiourea and ammonia such that the salt phase is eliminated as a source of loss, that the distillation of the raw mercaptoalkylsilanes takes place unimpaired by crystallizing guanidine hydrohalides, and that the formation of turbidity in the mercaptoalkylsilane distillates is eliminated.